1. Field of the Invention
The subject invention is directed to a mass flow meter, and more particularly, to a Coriolis effect mass flow rate meter for measuring small amounts of mass fuel flow to a fuel injector in a gas turbine engine. The subject technology may also be used as a gyroscope.
2. Description of Related Art
The next generation of gas turbine engines will include distributed flow architecture, wherein fuel flow to individual injector nozzles will be selectively modulated by an active fuel delivery control system. Such active distributed control system architecture requires individual mass flow rate control for each nozzle rather than the traditional single bulky flow meter near the main fuel tank. Thus, the mass flow sensors will need to be small while operating at small flow rates with fast response times. The estimated flow rates are expected to be about an order of magnitude less than those of conventional main flow stream meters.
By measuring small amounts, errors due to fuel property variation are magnified. For example, fuel can vary significantly from airport to airport and nation to nation as well as under varying weather conditions. Recalibration of the flow sensor to local conditions is not practical so it is desirable to design a flow meter that is insensitive to parameters such as fuel density and viscosity while still being accurate over a broad temperature range. Additionally, the flow meter must function under high pressure conditions without causing an undesirable pressure drop because gas turbine fuel lines may need to maintain a pressure of 1400 psi or more. Of course, the flow meter will also need high performance to match the response of fuel actuators. In view of the above, several flow meters have been developed to meet some of these criteria but none meet them all.
There are many existing methods to measure flow but only a few meet the criteria for being insensitive to local fuel qualities as follows: thermal anemometry that measures flow through heat transfer; an angular momentum-based torque measurement of flow; and Coriolis flow sensing of flow. Thermal anemometry is poorly suited to use across broadly varying temperature ranges. Torque measurement uses complex mechanical features that cannot be miniaturized. For example, U.S. Pat. Nos. 3,613,451 and 3,877,304, which are incorporated herein by reference in their entirety, disclose bulky torque measurement flow meters.
A Coriolis mass flow rate meter for measuring flow rates is disclosed in U.S. Pat. No. 4,127,028, which is incorporated herein by reference in its entirety. This flow rate meter is in the form of two vibrating U-shaped tubes. The U-shaped tubes also have the same natural frequency for longitudinal and torsoinal oscillations. Sensors are mounted away from the fixed ends of the tubes to detect the Coriolis effect. In order to increase the twist from the Coriolis effect, portions of the tubes are more widely separated in the area where the sensors are located. The Coriolis effect causes a difference between the signals of the sensors, such difference being proportional to the flow through the tubes.
Another Coriolis mass flow meter is disclosed in U.S. Pat. No. 6,564,650 (the '650 patent), which is incorporated herein by reference in its entirety. This flow meter 400 of the '650 patent uses dual loops 151, 152 formed from a single piece of tubing. An anchor 401 fixes the loops 151, 152 to a housing 450. A drive coil 131 causes the loops 151, 152 to oscillate in opposition to each other so that the assembly is balanced. Sensors 132, 133 determine the velocity of the loops 151, 152 during oscillation.
Although each of the Coriolis flow meters above can provide accurate measurements, each is relatively bulky and, thus, poorly suited to application on an individual injector nozzle. Additionally, applications require small flow rate measurements that typically dictate light and miniaturized flow meters; this is particularly true in aerospace applications where space and weight are primary considerations. The devices required for small flow rates present extraordinary manufacturing and performance difficulties.
Additionally, in high pressure applications, the tubes of the Coriolis flow meters are required to have thicker walls. The thicker walls significantly reduce the sensitivity or response to the Coriolis effect. In combination with the complete loop in which the ends are fixed, the resulting assembly becomes too rigid to perform well along with limiting the ability to create a compact/miniature flow meter. Thus, prior art Coriolis flow meters are poorly suited to adapt to high pressure, provide rapid response, and be utilized in small flow applications.